Pax Romana

The Pax Romana was a long period of relative peace and stability in the Roman Empire which lasted from its foundation by Augustus in 27 BC until the death of Marcus Aurelius in 180 AD. During the 207 years of peace, the empire achieved its greatest territorial extent and its population reached up to 70 million people, a third of the world's population. The period ended when Marcus Aurelius' son Commodus took the throne, after which the stability of the Empire devolved into anarchy and frequent civil conflicts.

Background
The key to the power of the Roman Empire was a stable, well-drilled professional army. Emperor Augustus (reigned 27 BC-14 AD) planned the final phase of its development. Coming to power through civil war, Augustus knew that the army was the foundation of his rule. He made the legions permanent formations with names and numbers. All legionaries had to be Roman citizens and he established a fixed term of service for soldiers, eventually set at 25 years. Augustus' army was 130,000 strong; auxiliaries equaled this number, giving a total army strength of around 260,000. Completion of service was rewarded with a grant of land. Non-citizen allies of Rome provided auxiliaries to fight alongside the legionaries. Almost all legionaries were stationed at permanent forts around the margins of the empire. The Praetorian Guard was often the force that made or unmade emperors in power struggles.

History
The last phase of expansion of the Roman Empire occurred in the period between the reign of Augustus (27 BC-14 AD) and the end of the reign of Emperor Trajan in 117 AD. The Romans were tempted to cross the long land frontiers of their provinces in Western and Southern Europe and to press further into Asia from their provinces in the eastern Mediterranean. They also traversed the English Channel to occupy much of Great Britain.

Although the Roman legions were a formidable ﬁghting force, the campaigns that they were engaged in were far from supplying the empire with an unbroken string of successes. The battles they fought often revealed their vulnerability when faced with enemies employing very different tactics on what was to the Romans alien terrain. An early reminder of the limits of Roman power was provided by a defeat at the hands of the Germanic tribes led by Arminius, a Chieftain of the Cherusci, who lived in the area of present-day Hanover. Arminius was commanding a body of auxiliaries ﬁghting for the Romans, while secretly forming a tribal alliance to oppose them. At the Teutoburg Forest in 9 AD, three legions under Publius Quinctilius Varus were ambushed by Arminius' forces and massacred. The Romans had their revenge through punitive expeditions led by Germanicus between 14 and 16 CE, but the tribes across the Rhine were never subdued as the Gauls had been.

Romans in Britain
The conquest of Britain began under Emperor Claudius in 43 AD. Rome had exercised an inﬂuence over Britain's Celtic peoples ever since Julius Caesar's two expeditions across the Channel almost a century earlier. There were puppet kingdoms that paid tribute to Rome, and Roman support for an ousted allied king provided an easy pretext for military occupation. Four legions ferried across from Boulogne sufﬁced to overcome initial resistance in the southeast, but Caratacus, a Chieftain of the Catuvellauni tribe, escaped to continue the ﬁght further west. By 47 AD the whole of southern England was under Roman rule, although resistance in Wales continued despite the eventual capture and execution of Caratacus. The Romans were distracted from warfare in Wales in 60 AD by a major revolt in eastern England, led by Boudicca of the Iceni tribe. After this had been suppressed, it took another 16 years for all of Wales to be brought to heel. Conquest to the north proved more problematic. The Romans advanced into Caledonia (modern-day Scotland) and won a victory at Mons Graupius in 84 CE, but the tribes of the region escaped their long-term rule. When Hadrian's Wall was built as a defensive perimeter to the British province in 122 AD, it was much further south than the previous wall than they built in Scotland. In 142 AD, the Antonine Wall, built along the line of the Clyde and the Forth rivers in central Scotland, was the Roman empire's most northern frontier in Britain; it took 12 years to build, but was held for only 20 years.

Extending the Empire
Rome’s preoccupation with maintaining and extending its imperial frontiers was occasionally disrupted by disturbances within the empire. In 66 AD the province of Judaea rose in revolt. Vespasian, an experienced military commander who had taken part in the invasion of Britain in 43, was sent to suppress the revolt, but his campaign was interrupted by an outbreak of civil war that followed the death of Emperor Nero in 68. After legion once more fought legion, as in the days of Caesar and Pompey, Vespasian emerged as emperor. He appointed his son, Titus, to continue the war in Judaea. In 70 AD Jerusalem was taken by the Roman army after a long siege, its temple was destroyed and the revolt effectively ended. A small Jewish group held out in the hill fortress of Masada until 73 aD. The taking of Jerusalem and Masada were both classic examples of Roman siege warfare, with the use of rams, ballistas, and siege towers. At Masada, on the coast of the Dead Sea, the building of an immense ramp was required to bring the siege engines up the mountainside. In neither case did the city's defenders stand a chance in the face of the Roman forces; at Masada the population ﬁnally committed suicide to avoid capture.

In 98 AD the Roman empire came under the rule of an exceptional military leader, the Spanish-born Emperor Trajan. He indirectly owed his rise to power to King Decebalus of Dacia (roughly modern-day Romania) who had refused to be cowed by Roman military campaigns under Emperor Domitian. The subsequent fall of the humiliated Domitian allowed Trajan to emerge as emperor, and dealing with Dacia in order to reassert Roman authority in the region was thus the ﬁrst item on his agenda. Trajan fought two campaigns in Dacia. The ﬁrst in 101-102 was followed by a peace that left Decebalus on the throne as a puppet ruler. When the Romans left, Decebalus rebuilt his army and began to show signs of causing trouble again, so in 105-106 Trajan returned, assaulted and laid waste the Dacian capital, Sarmizegetusa, and absorbed Dacia into the Roman empire. Trajan's Column, erected to mark this triumph, records exceptional feats of engineering, such as bridge building, accomplished by the legions, as well as the defeat and enslavement of the Dacians.

In 114, when Trajan was over 60 years old, he embarked upon another remarkable military venture in the east. The Parthians, rulers of Persia and Mesopotamia who fought chieﬂy as mounted archers, had inﬂicted a notorious defeat on the Romans at Carrhae in 53 BC and remained a potential threat to Rome's eastern provinces. On the pretext of a disagreement over Armenia, but out of a desire above all for military glory, Trajan invaded Parthian territory in force, overrunning Mesopotamia, reaching the Persian Gulf, and capturing the Parthian capital, Ctesiphon. When Trajan died of natural causes in 117, the Roman empire had reached its greatest extent. Trajan's gains in Mesopotamia could not be sustained. Under his far less warlike successors, Hadrian and Antoninus Pius, the legions pulled back to defensible borders. Marcus Aurelius (ruled 161—180) fought many wars, but they were mostly defensive actions against renewed pressure on the borders from Germanic and Parthian aggression.

Aftermath
In the 3rd century AD the Roman empire almost disintegrated. Barbarian invaders overran the frontiers as rival claimants battled for the imperial throne. The empire was saved from collapse by the Emperor Aurelian (reigned 270-275), who restored unity, defeated the Goths, and endowed Rome with defensive walls. In 293 Emperor Diocletian devised a tetrarchy, four co-rulers, to defend the over-large empire. Two senior emperors (with the title Augustus) reigned together, each assisted by a junior emperor (with the title Caesar). The four rulers acted as military commanders, based near vulnerable frontiers rather than in Rome. This system allowed the empire to reassert its military strength, with, in particular, a rare victory over the Sassanids, the successors to the Parthians in Persia. The tetrarchy collapsed after Diocletian's abdication in 305. This led to another period of civil war as claimants to the title of Augustus proliferated.